The process of converting (or transforming) one form of energy into another is often referred to as transduction. A transducer is a device that is typically employed to perform such a function, and transducers can be characterized by the direction in which physical system (e.g., pressure, temperature, sound waves, etc.) passes through them. For example, a sensor is a type of transducer that receives and responds to a signal/stimulus from a physical system (e.g., temperature) and produces an electrical signal that represents information about the physical system. An actuator, on the other hand, is a transducer that controls/generates a physical system (e.g., sound waves), in response to some electrical signal. For example, a speaker transforms an electrical signal of a recording to mechanical sound waves.